Our 3-grade anatomic classification of pulmonary arterial changes for patients with pulmonary arterial hypertension is a modification of the classification of Health and Edwards. Our new classification consists of medial thickening only, grade I; medial and intimal thickening, grade II; and both medial and intimal thickening plus plexiform lesions, grade III. These anatomic changes in the pulmonary arteries result entirely from pulmonary hypertension. Intimal thickening without medial thickening, however, is observed in older persons, and, in itself, is not a consequence of pulmonary hypertension. Observations in our 87 necropsy patients over age 15 with major congenital cardiovascular malformations indicate that the changes in the pulmonary arteries are directly related to the level of pulmonary arterial pressure. Direction of the shunt, location of the shunt, and age of the patient were important only by their effect on the pulmonary arterial pressure. The patients with PDA had the most severe changes in the pulmonary arteries and the highest average pulmonary arterial pressures. In patients with pulmonary hypotension (isolated PS) and in patients with PS and VSD (TF), the media of the pulmonary arteries is thinner than normal and the lumens are larger than normal. Intimal thickening is usually seen in patients with TF even though the media is not thickened. Surgical creation of systemic-to-pulmonary arterial anastomoses may cause the pulmonary arterial media to thicken to normal widths or even to widths thicker than normal, but the development of pulmonary hypertension after such operations is infrequent.